The present invention relates to a pumping unit for feeding fuel, preferably diesel fuel, from a storage tank to an internal combustion engine.
In particular the present invention relates to a pumping unit of the type comprising a fuel storage tank; a high-pressure pump, in the case in question a piston pump, for feeding the fuel to an internal combustion engine; a pre-feed pump, in the case in question a gear pump, for feeding the fuel from the storage tank to the piston pump; and a hydraulic circuit for connecting together the storage tank and the internal combustion engine.
Operation of the gear pump and the piston pump is actuated by a transmission shaft comprising a first portion extending through a pump body of the piston pump and a second portion extending through a pump body of the gear pump.
The hydraulic circuit comprises a first branch for connecting together the storage tank and the gear pump; a second branch, which connects together the gear pump and the piston pump and extends through the pump body of the piston pump so as to lubricate the first portion of the transmission shaft; and a third branch for connecting together the piston pump and the internal combustion engine.
The hydraulic circuit further comprises a filtering device arranged along the second branch, for filtering the fuel fed to the pump body of the piston pump.
The fuel leaks resulting from the lubrication of the first portion of the transmission shaft are discharged into the storage tank via a fourth branch of the hydraulic circuit.
The fourth branch is connected to a delivery of the gear pump and is configured so that fuel trickling between the first portion of the transmission shaft and the associated support bearings has a pressure lower than the pressure of the fuel at the delivery of the gear pump. Consequently, the second portion of the transmission shaft is lubricated with the fuel supplied from the delivery of the gear pump, namely with a fuel flow directed from the delivery of the gear pump to the fourth branch.
The known pumping units of the type described above have a number of drawbacks mainly arising from the fact that the second portion of the transmission shaft is lubricated with the fuel from the delivery of the gear pump, namely with fuel drawn from the second branch of the hydraulic circuit ahead of the filtering device and, therefore, not yet filtered.
Consequently, any solid particles present in the fuel used to lubricate the second portion of the transmission shaft are fed between the pump body of the gear pump and the second portion of the transmission shaft causing wear of these components due to abrasion.